Aromatic carboxylic acids are highly useful organic compounds. Some are used as intermediates for the preparation of other organic compounds, and some are used as monomers for the preparation of polymeric materials. For example, terephthalic acid is used to prepare polyethylene terephthalate, a widely used polyester material and the naphthalenecarboxylic acids (i.e. the naphthoic acids) are used for preparing photographic chemicals and dyestuffs. Additionally, naphthalenedicarboxylic acids can be used to prepare a variety of polyester and polyamide compositions. One such naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, is a particularly useful aromatic carboxylic acid because it can be reacted with ethylene glycol to prepare poly(ethylene-2,6-naphthalate) (PEN). Fibers and films manufactured from PEN display improved strength and superior thermal properties relative to other polyester materials such as polyethylene terephthalate. High strength fibers made from PEN can be used to make tire cord, and films made from PEN are advantageously used to manufacture magnetic recording tape and components for electronic applications.
In order to prepare high quality PEN most suitable for the aforementioned applications, it is desirable to use purified 2,6-naphthalenedicarboxylic acid. Similarly, it is desirable to use purified forms of other aromatic carboxylic acids when using these compounds for the hereinabove mentioned applications.
Aromatic carboxylic acids, and particularly 2,6-naphthalenedicarboxylic acid, are conveniently prepared by the liquid phase, metal catalyzed oxidation of an alkyl or acyl substituted aromatic compound. During this oxidation, the alkyl group (for example a methyl, ethyl or isopropyl group) or acyl group is oxidized to a carboxylic acid group. Although this is an effective oxidation reaction it nevertheless has some drawbacks. For example, when a 2-alkyl or 2-acyl substituted naphthalene compound is oxidized, the naphthalene ring itself is susceptible to oxidation and trimellitic acid is produced. Incomplete oxidation of a methyl group produces an aldehyde group instead of a carboxylic acid group. Furthermore, when a promoter such as bromine is used during the liquid phase oxidation, brominated aromatic carboxylic acids are produced. Although all of these impurities are undesirable, trimellitic acid is particularly troublesome because it tends to complex tightly to the metal oxidation catalysts. Such complexed metal is difficult to remove from the aromatic carboxylic acids and, additionally, any process streams containing trimellitic acid are not readily returned to the oxidation reaction mixture because the trimellitic acid complexes to and consequently deactivates the oxidation metal catalysts. Such a recycle stream may originate from oxidation reaction mixture mother liquor that is separated from the aromatic carboxylic acid after the alkyl or acyl group is oxidized. Such recycle stream can also be a wash stream that is formed by washing the aromatic carboxylic acid with a suitable solvent. Thus, a method that provides for an aromatic carboxylic acid having a reduced level of trimellitic acid and/or other impurities, or that reduces the level of trimellitic acid in a process stream so that it can be more effectively recycled to the oxidation reaction mixture is desirable. The present invention provides such a method.